cmd/dist: add serial/small build option for GOMAXPROCS=1
cmd/dist: add serial/small build option for GOMAXPROCS=1

If GOMAXPROCS=1, force serial execution, which is better for
debugging build problems and also minimizes footprint, if that
happens to matter.

This wasn't good when the bootstrap was 1.4 because there
default GOMAXPROCS=1, but that is no longer the bootstrap
version.

Change-Id: I637e25c8acb4758795fceef63921eda359a7be29
Reviewed-on: https://go-review.googlesource.com/c/go/+/452556
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go